In radio frequency systems and platforms supporting transmission/reception systems, it is known practice to use a system for identifying a friendly or authorized user on specific frequencies (principle of friend-foe interrogators IFF, which stands for “Identification Friend and Foe”). A transponder of this type produces an external tagging in that it responds to an interrogation recognized as originating from an authorized user, by returning a response in the form of a signal coded specifically for said authorized receivers.
In these systems, the major drawbacks lie in the fact that the frequency plans supporting the interrogation/response interchanges are fixed and known, and in that the signals can be fairly easily detected and analyzed by third party receivers. For example, the IFF transponders, even in their protected versions (mode S or mode 5 known to those skilled in the art), receive the interrogation on a known and standardized frequency (1030 MHz), and respond on a frequency that is also known and standardized (1090 MHz). The 1030 and 1090 MHz frequencies can thus be easily surveyed, analyzed, even disrupted by third parties (interferences).
Also, it is necessary to have transmitter systems specific to the identification, operating in bands that are also specific and generally very different from the useful signal transmission bands, hence an increase in indiscretion, an increased vulnerability to third parties, an increase in the number of hardware components, increased risks for electromagnetic compatibility, etc.
Regarding the modern digital radio communication networks, it is also known practice to include frame words, synchronization sequences and pilot signals in the forms of FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) waves or in the OFDM (Orthogonal Frequency Division Multiplexing) coding modulations which enable a user to detect the host networks and be synchronized on infrastructure transmitters, on relay terminals or on communication nodes. The tagging of the transmitter is produced by the inclusion of these words, sequences or pilot signals, and by coded messages in the broadcast signaling to which the detection of these words, sequences or pilot signals gives access. This tagging is then internal to the useful signal.
The systems and methods known from this prior art notably have the drawback of requiring wave forms and access protocols that are complex, and all the more complex when protection is sought for these words, sequences, pilot signals and signaling messages with respect to unauthorized third parties. These methods therefore apply only to certain restricted categories of transmission systems (typically communication infrastructures and nodes in the digital networks). Furthermore, they consume a significant share of the radio frequency resource allotted for the transmissions and for the overall operation of the networks which implement them, at the expense of the capacity of the transmitters and of the network. Finally, when the words, sequences, pilot signals and signaling messages have to be protected with respect to unauthorized third parties, the wave forms and corresponding access protocols are made much more complex.
The patent application U.S. 2010/0246825 corresponds to a technique (well known) for authenticating transmitted messages as used in the modern cryptographic and integrity control methods.
The patent application FR 2 629 655 A1 relates to the effective friend-foe identification and describes a system using an identification signal which has a level lower than the other signals.
The document by KLEIDER J E ET AL: “Radio frequency watermarking for OFDM wireless networks”, frequency watermarking for OFDM wireless networks“, ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, 2004. PROCEEDINGS. (ICASSP '04). IEEE, H04L9/32 INTERNATIONAL CONFERENCE ON MONTREAL, QUEBEC, CANADA MAY 17-21, 2004, PISCATAWAY, N.J., USA, IEEE, PISCATAWAY, N.J., USA, H04H60/35 vol. 5, May 17, 2004 (05-17-2004), pages H04B1/59397-400, XP010718949, discloses a Radio Watermarking method.
For the rest of the description, the following concepts are defined:                a useful signal, hereinafter denoted Su, is a main radio frequency signal originating from a transmitter of any type (infrastructure or node of a communication network; terminal; channel sounders; transmission systems used for controlling communications in a network, generating or not generating interferences; etc.); thus, in the context of the invention, an interference signal will be able to be considered as a useful signal and denoted Su.        a tag signal or tagging signal, hereinafter denoted Ctag, is a secondary signal transmitted at the same time and on the same frequency bands as the useful signal, but generally with lower levels, and which exhibits parameterizations and/or which contains information that can be recognized by authorized users for the purposes of identification of the transmitter of the useful signal by authorized third parties.        